battery / volts

You are past that point now.

Running the voltage down that low will damage the battery, do it too often and it's had it.

Most voltage cut outs operate around 11.3 volts so unless you voltmeter is wrong then there may be a fault in the cut out system.

As I understand it you need to charge them when they drop to no less that about 10.4 volts or risk damaging them.
Across the terminals is a no load voltage.
What you need to do is put the load on them then test them

Hi Shane
ONE good rule for lead acid batterries IS keep them as fully charged as posible.[12.6/12.7v with no charge ]
#2 do not discharge below 50/60% remaining capacity for long life[ approx 12,5v with no load & depending on load about 12 .2v onload]
IE WORK THEM BETWEEN 12.7V &12.2 V FOR BEST LIFE .
all reading @ battery terminls.

11.3 is close to FLAT, TAKING THEM THAT LOW WILL DRAMATICALY SHORTEN THEIR LIFE.
Do not let them stand not fully charged as they will sulphate up causing loss of capacity & shorten life

It doesn't work that way.

If these batts are AGM's, fully charged resting voltage should be around 13v, a 50% discharged AGM will read around 12 volts. If it gets down to 6 volts, it's usually scrap (but not always).

Keeping a DC battery charged and above 50% will ensure maximum life span.

Hi all,

Can anybody please tell me how you can determine the charge of a battery between fully charged and % flat measuring volts ?

The following link shows discharge curves of various cells. A lead acid battery starts at 2.2 volts (approx) when fully charged and is almost a constant voltage across the discharge curve until the final sudden drop when flat.

http://www.mpoweruk.com/performance.htm

All I can tell, because of the different Characteristic of cell construction is if a battery is on charge or being discharged when using a volt meter. Volts does not equate to % of charge.


Redeye

About one year ago I made a post to this forum with some information gained from professional testing I am doing on AGMs - try searching on something like "harding AND agm AND 2%".

The basic rule for AGMs is... discharge them as little as possible before recharging.

They should not be taken below 10V5 (some manufacturers do spec down to 9V0 but this is specialised stuff - not for domestic use).

From memory of the figures I posted: once an AGM reaches 11V0 it only returns a further 2% of fully charged capacity before it hits 10V5.

At 11V5 it will return 20% of capacity before it hits 10V5.

Conclusion: it is never worth discharging below 11V0 and you should only go below 11V5 infrequently. In general try to recharge when the battery falls to about 12V0 to 12V3.

Mike Harding

We are often asked how do you know what power is left in a battery, condition or state of charge. (SOC)

Here is a simple graph that shows SOC vs. voltage.

Please note this is an average and there are slight variables that are effected by temperature and battery type.

As the above graph shows....keep it above 12V ( no load or under load ) and you really can't go wrong..

If you are regularly going too far down past 12V, then you probably need more battery power ..

Hi Shane, your battery was around 10% capacity if you had an 11.3 volt reading with a load, providing that load is no more than what your fridge pulls while the compressor is running.

Your No-Load reading is next to useless because to get anywhere near an accurate No-Load leading your battery needs to be sitting unused for at least 12 hours and even after 48 hours you still need to factor in a 5% error margin.

As Mike DID posted above, using something like a winch, because of the high current draw, will give you a reading of something like 9 volts with a battery that still has a 50% capacity, but if you are like most people out camping and wanting to keep an eye on the State of Charge ( SoC ) of your auxiliary battery(s), do your reading while the fridge is running, because most fridges only draw a few amps while the compressor is on, you will get a much more accurate and instant indication of the SoC of your battery than you will ever get while measuring the battery in a No-Load ( Correctly known as an Open Circuit ) voltage measurement.

The voltages shown in the posts above are Open Circuit voltage readings and in all fairness, are next to useless for anyone trying to get an idea of the SoC of their batteries while the batteries are in day to day use while out camping.

The voltages below are the sort of readings you can expect to get while your fridge is running.

100% 12.70

90% 12.50

80% 12.42

70% 12.32

60% 12.20

50% 12.06

40% 11.90

30% 11.75

20% 11.58

10% 11.31

0% 10.50

As your battery voltage reading was at 11.3 and as Mike Harding posted above, you had very little capacity left.

One more point, many fridges, with an automatic low voltage cut out operation, will do so at about 10.5 volts and if you have ever checked your battery’s voltage after the fridge has cut out, it is not unusual to get a reading from 10.5 to well over 11 volts. Again, to get anywhere near an accurate idea of the true SoC of your battery(s) take your reading while the fridge is running.

Hi Shane
The problem with volt readings are:-
#1:- How accurate is the voltmeter ,I have seen errors of up to .5v[ halfvolt] with the normal expanded scale ones available from super cheap etc.A high accuracy digital meter is needed to read the small voltage variations involved.or you need to know what the error is on yours.
#2:- Where is the voltmeter connected, If it has it's own DIRECT connection to the battery[ unlikely],it will read bat volts both on load & noload.
It will most likely be connected to the battery via some load carrying wiring which WILL give an error depending on the current the wiring is carrying. ie A VARIABLE ERROR.
The on load voltage of a battery depends on
#1 :-The state of charge of the battery
#2:-The amps being drawn from the battery high amps causes a a lower reading WHILE THE AMPS ARE FLOWING ,LOWER AMPS WILL GIVE LESS DROP[ HIGHER VOLTS ] This is due to the internal resistance of the battery.
So you can see from this that the onload volts test is subject to a number of variables which would have to be taken into account.

In my opinion, apart from the open circuit reading taken after 24hrs @ noload & nocharge, the next best test is with a very light load, allowing for about .2 volts drop from open circuit readings.
It is however only an approximate indication of state of charge so maintain a safe margin for long battery life.I would not go below 12v preferably 12 .2 v.
If your batteries are flooded wetcell s ie have caps, then a hydrometer is the BEST available, to YOU. means of testing state charge ,& individual cells for possible faults developing etc